Acids, Bases & Acids, Bases & BuffersBuffers

IntroductionIntroduction

• Sourness in foods is caused by acids, molecules that release protons.

• The chemical opposite, bases, are all around us.

Some Substances that are AcidsSome Substances that are Acids

FoodsVinegar Mustard Pickle Juice (all contain acetic acid

HC2H3O2)

Oranges Lemons Grapefruits Gelatins Most sodas(all contain citric acid H3C6H5O7)

Sour Milk Buttermilk Yogurt (all contain lactic acid HC3H5O3)

Cranberry Juice (contains benzoic acid HC7H5O2)

Apples Watermelons Grapejuice Wine (all contain malic acid H2C4H4O5)

Spinach Tomatoes (both contain oxalic acid H2C2O4)

Grapejuice Wine (both contain tartaric acid H2C4H4O6)

All acids above are ORGANIC ACIDS --acids with lots of C atoms. (Most are weak acids)

Some Substances that are AcidsSome Substances that are Acids

Other Common AcidsSTRONGSTRONG WEAKWEAK

Hydrocloric acid (HCl) Carbonic Acid (H2CO3)

Nitric Acid (HNO3) Phosphoric Acid (H3PO4)

Sulfuric acid (H2SO4) Hydrofluoric Acid (HF)

Perchloric Acid (HClO4) Hydrocyanic Acid (HCN

Hydrobromic acid (HBr) Sulfurous acid (H2SO3)

Hydroiodic Acid (HI)

How do we recognize an acid by its chemical formula? Ans: There is an H or H’s at the fore of the formula

Some Substances that are BasesSome Substances that are Bases

Household Products

Household Ammonia (contains ammonia (NH3) in water)Drain and Oven Cleaners (contains sodium hydroxide, NaOH)

Antacids (Rolaids, Tums, Alka Seltzer, Maalox (contains Al(OH)3 )

Milk of Magnesia (contains Mg(OH)2 )Baking Soda (contains NaHCO3 )

ToothpasteSoap

Most cleaning products are basic (a few glass cleaning products contain acetic acid or HCl)

Some Substances that are BasesSome Substances that are Bases

Other Common BasesSTRONGSTRONG WEAKWEAK

Lithium hydroxide (LiOH) Ammonia (NH3)Sodium hydroxide (NaOH) Amines (e.g

CH3CH2NH2, (CH3)2NH, and C5H5N)

Potassium hydroxide (KOH)Cesium hydroxide (CsOH)Calcium hydroxide (Ca(OH)2)Barium hydroxide (Ba(OH)2 )

Amines are ORGANIC BASES

How can we generally recognize a base by its chemical formula? Ans: OH or OH’s at the end of the formula & a metal at the beginning of the formula.

General Properties of Aqueous Acids and BasesGeneral Properties of Aqueous Acids and Bases

Aqueous means dissolved in water

Acids have a sour taste Bases have a bitter taste

Acids are electrolytes Bases are electrolytes

(Electrolytes conduct electricity)

Definitions of Acids & BasesDefinitions of Acids & Bases

Arrhenius

Acid: H+ or H3O+ producer in water (ex: HCl, HNO3, H2SO4)

Base: OH- producer in water (ex: NaOH, LiOH, Ca(OH)2)

Bronsted-Lowry

Acid: proton (H+) donor (ex: NH4+, HSO4

-, all Arrhenius acids)

Base: proton acceptor (ex: NH3, H2O, CO32-)

Lewis

Acid: electron-pair acceptor (ex: H+, positive ions --Al3+, Fe2+)Base: electron pair donor (ex: OH-, negative ions– F-, O2-, N3-)

An Acid-Base Reaction

Neutralization is the reaction of an H+ (H3O+) ion from the acid and the OH - ion from the base to form water, H2O.

The neutralization reaction is exothermic and releases approx.56 kJ per mole of acid and base.

H+(aq) + OH-

(aq) H2O(l) H0rxn = -55.9 kJ

Strong and Weak Strong and Weak Acids & BasesAcids & Bases

Strong acid: HA(g or l) + H2O(l) H2O+(aq) + A-(aq)

The Extent of Dissociation for Strong Acids

Complete dissociation

Weak acid: HA(aq) + H2O(l) H2O+(aq) + A-(aq)

The Extent of Dissociation for Weak Acids

Partial Dissociation

Strong & Weak Acids

Strong acids dissociate completely into ions in water:

HA(g or l) + H2O(l) H3O+(aq) + A-

(aq)

Nitric acid is an example: HNO3 (l) + H2O(l) H3O+(aq) + NO3

-(aq)

Weak acids dissociate very slightly into ions in water:

HA(aq) + H2O(aq) H3O+(aq) + A-

(aq)

In a dilute solution of a weak acid, the great majority of HA molecules are undissociated:

In the same manner, bases are considered either strong or weak.

Classifying the Relative Strengths of Acids and Bases

Qualitative Classifications Qualitative Classifications (memorize these)Strong acids. There are two types of strong acids: 1. HCl, HBr, and HI 2. Acids containing O in which the # of O atoms exceeds the number of ionizable H atoms by two or more, such as HNO3, H2SO4, HClO4

Weak acids. Many more weak acids than strong ones. Four types are: 1. HF 2. Those acids in which H is not bounded to O or to a halogen, such

as HCN and H2S 3. Acids containing O in which the # of O atoms exceeds by one the number of ionizable H atoms, such as HClO, HNO2, and

H3PO4

4. Organic acids (general formula RCOOH), such as CH3COOH and C6H5COOH

Classifying the Relative Strengths of Acids and Bases

Qualitative Classifications Qualitative Classifications (memorize these)Strong bases. Soluble compounds containing O2- or OH- ions are strong bases. The cations are usually those of the most active metals: 1) M2O or MOH, where M= Group 1A(1) metals (Li, Na, K, Rb, Cs) 2) MO or M(OH)2, where M = Group 2A(2) metals (Ca, Sr, Ba) [MgO and Mg(OH)2 are only slightly soluble, but the soluble portion dissociates completely.]

Weak bases. Many compounds with an electron-rich nitrogen are weak bases (none are Arrhenius bases). The common structural feature is an N atom that has a lone electron pair in its Lewis structure. 1) Ammonia (:NH3) 2) Amines (general formula RNH2, R2NH, R3N), such as CH3CH2NH2, (CH3)2NH, (C3H7)3N, and C5H5N

The Hydronium Ion, The Hydronium Ion, HH33OO++

H+ comes from acid and combines with H2O to yield H3O+, the hydronium ion

H3O+ Concentration, [H3O+]

• The acidity of a solution is normally specified by the concentration of H3O+ in moles per liter of solution, M

– Strong acids: acid concentration = [H3O+]

– Weak acids: acid concentration > [H3O+]

• Pure water has a [H3O+] = 1 x 10-7 M.

Autoionization of Water

H2O(l) + H2O(l) H3O+(aq) + OH-

(aq)

Kc =[H3O+][OH-]

[H2O]2

The ion-product for water, Kw:

Kc[H2O]2 = Kw = [H3O+][OH-] = 1.0 x 10-14 (at 25°C)

For pure water the concentration of hydroxyl and hydronium ions must be equal:

[H3O+] = [OH-] = 1.0 x 10-14 = 1.0 x 10 -7 M (at 25°C)

The molarity of pure water is: = 55.5 M1000g/L18.02 g/mol

Acid and Base Character and the pH Scale

H+(aq) + H2O(l) H3O+

(aq)[H+] = [H3O+]

A scale called the pH scale is used express the H+ ion conc’n in water:

pH = - log[HpH = - log[H33OO++]]What is the pH of a solution that is 10-12 M in hydronium ion ?

pH = -log[H3O+] = (-1)log 10-12 = (-1)(-12) = 12

What is the pH of a solution that is 7.3 x 10-9 M in H3O+ ?pH = -log(7.3 x 10-9) = 8.14

pH of a neutral solution = 7.00 [H3O+] = [OH-]pH of an acidic solution < 7.00 [H3O+] > [OH-]pH of a basic solution > 7.00 [H3O+] < [OH-]

Relationship b/w [H3O+] and [OH-] and the relative acidity of solutions

The pH Values of Some Familiar Aqueous Solutions

pH + pOH = 14pH + pOH = 14

For every change of 1 uniton the pH scale, [H3O+ ] changes by a factor of 10.

Calculating [H3O+], pH, [OH-], and pOH

Problem ydiydi

A chemist dilutes concentrated hydrochloric acid to make two solutions: (a) 3.0 M and (b) 0.0024 M. Calculate the [H3O+], pH, [OH-], and pOH of the two solutions at 25°C.

Calculating [H3O+], pH, [OH-], and pOH

Plan: We know that hydrochloric acid is a strong acid, so it dissociates completely in water; therefore [H3O+] = [HCl]init.. We use the [H3O+] and Kw to calculate the [OH-] and pH as well as pOH.

Solution: (a) [H3O+] = 3.0 M pH = -log[H3O+] = -log(3.0) = -0.477

[OH-] = = = 3.333 x 10-15 M Kw

[H3O+]1 x 10-14

3.0

pOH = - log(3.333 x 10-15) = 14.477

(b) [H3O+] = 0.0024 M pH = -log[H3O+] = -log(0.0024) = 2.62

[OH-] = = = 4.167 x 10-12 M

pOH = -log(4.167 x 10-12) = 11.38

Kw

[H3O+]1 x 10-14

0.0024

pH (indicator) paper

pH meter

Methods for Measuring the pH of an Aqueous Solution

Acid Rain: Fossil Fuel CombustionAcid Rain: Fossil Fuel Combustion• SO2 and NO2 formed during fossil fuel combustion combine

with atmospheric water to form acid rain.

• Unpolluted rain is slightly acidic due to atmospheric carbon dioxide.

• Most acidic rainfall occurs in the northeastern U.S.

Acid Rain: The EffectsAcid Rain: The Effects

• The environment into which acid rain falls determines its fate.

• In some cases naturally occurring geography can serve to neutralize the acid.

• Rapid acidification occurs when neutralization is not possible.

• Lakes and Streams– Approx. 2000 lakes and streams in the eastern U.S.

have elevated pH.– Some aquatic species cannot survive.– U.S. emissions have contaminated Canadian lakes.

• Building Materials– Acids dissolve stone, marble, paint– Rusting of steel is accelerated

• Forests and Reduced Visibility– Trees cannot grow and fend off disease.– Sulfate aerosols account for 50% of visibility

problems in the eastern U.S.

Clean Air Act AmendmentsClean Air Act Amendments

• Cut SO2 emissions to half of 1980 levels by 2010– Use low-sulfur coal, rremove sulfur before burning– Use flue gas scrubbers– Conservation and efficiency of customers

• SO2 emission allowances

– Can be traded among utilities but congress reduces number of allowances as per regulations

What is a Buffer ?What is a Buffer ?

A solution that changes pH only slightly small amounts of strong acid or base are added to it .

The Effect of Addition of Acid or Base to The Effect of Addition of Acid or Base to Unbuffered or Buffered Solutions Unbuffered or Buffered Solutions

pH of unbuffered (top) changes by a large amount; pH of buffered (bottom) changes only slightly

3 Kinds of Buffer Solutions• Acid Buffer: a solution of weak acid & its salt

(conjugate base); its pH < 7.Examples

HNO2/NO2- ; HClO2/ClO2

- ; CH3COOH/CH3CO2-

• Base Buffer: a solution of weak base & its salt (conjugate acid); its pH > 7.

Examples

NH4+/NH3 ; H2PO4

- / HPO42 - ; (CH3)3NH+/ (CH3)3N

• Neutral Buffer

Why are Buffers Important ?Why are Buffers Important ?• Human blood & other cell fluids buffered at pH =

7.40 ± 0.05. A rise in pH above 7.45 (alkalosis) or a drop in pH below 7.35 (acidosis) can be life-threatening. (Intravenous solutions are buffered)

• Some chemical reactions can only be done within a narrow pH range;

• pH meters have to be calibrated;

• Culturing (growing in prepared medium) of bacteria done in specific small pH range;

• Largest buffered system-the OCEAN-relies on presence of bicarbonates (HCO3) and silicates.

How does a buffer resist How does a buffer resist pH changes?pH changes?

How a Buffer Works

• Small quantities of H3O+ (acid) or OH- (base) added to buffer cause a small amount of one buffer component to convert into the other. • As long as amounts of H3O+ and OH- are small compared

to conc’ns of acid and base in buffer, added ions will have little effect on the pH since they are consumed by buffer components.